Method of rock cutting employing plasma stream

ABSTRACT

A method especially useful for cutting and breaking hard rock such as granite from the face of a tunnel is disclosed. A pattern of slots are cut into the rock face by directing a high velocity plasma jet on the rock face to melt a portion of the rock face and produce a molten film and applying electrical power to the plasma-jet and a cooperating electrode to flow electric current through the molten film to further heat the molten film and melt additional rock to form a slot. After the pattern of slots are formed, spaced plasma streams are introduced into the slots and electrical power of a frequency effective to produce dielectric heating in the rock is applied through the plasma streams to produce a heated region within the rock mass which thermally stresses and severs that rock mass portion into fragments.

United States Patent [191 Thorpe Jan. 29, 1974 METHOD OF ROCK CUTTINGEMPLOYING PLASMA STREAM [75] Inventor:

[52] US. Cl 299/14, 175/16, 219/75 [51] Int. Cl. E2lc 37/18 [58] Fieldof Search 299/14; 175/16, 13; 219/75 [56] References Cited UNITED STATESPATENTS 10/1961 Karlovitz l75/l6X 4/1966 Margiloff 299/14 PrimaryExaminer-Ernest R. Purser Attorney, Agent, or FirmWillis M. Ertman I 57ABSTRACT A method especially useful for cutting and breaking hard rocksuch as granite from the face of a tunnel is disclosed. A pattern ofslots are cut into the rock face by directing a high velocity plasma jeton the rock face to melt a portion of the rock face and produce a moltenfilm and applying electrical power to the plasma-jet and a cooperatingelectrode to flow electric current through the molten film to furtherheat the molten film and melt additional rock to form a slot. After thepattern of slots are formed, spaced plasma streams are introduced intothe slots and electrical power of a frequency effective to producedielectric heating in the rock is applied through the plasma streams toproduce a heated region within the rock mass which thermally stressesand severs that rock mass portion into fragments.

22 Claims, 4 Drawing Figures METHOD OF ROCK CUTTING EMPLOYING PLASMASTREAM SUMMARY OF INVENTION This invention relates to methods forcutting and breaking hard rock and more particularly to methodsespecially useful for cutting and breaking hard rock such as granitefrom the face of tunnel. Boring a tunnel through hard rock has been doneconventionally by drilling a number of holes into the tunnel face withpercussive tools and then fragmenting the tunnel face by the detonationof explosives inserted into the drilled holes. The operation has beenslow and has required expensive capital equipment. A technique has beendeveloped for fragmenting rocks by dielectric heating, but it has beendifficult to establish reliable electrical contact with the rock masswith the electrodes used and the positioning of the electrodes has beenslow and cumbersome. Objects of the invention include the provision ofmethods permitting more rapid tunnelling and less expensive tunnellingthrough hard rock.

Another object of the invention is to provide novel and improved methodsfor cutting rock.

Another object of the invention is to provide a reliable, high speedmethod for hard rock cutting.

One aspect of the invention features the method of rock cutting, e.g.for extending a tunnel face through a rock face comprising the steps ofcutting a pattern of slots into the rock face, each slot being cut bydirecting a plasma-jet on the rock face to melt a portion of the rockface and produce a molten film thereon, and applying electrical power tothe plasma-jet and a cooperating electrode to cause electric currentalong a path that includes the plasma-jet, the molten film, and thecooperating electrode, thus further heating the molten film and meltingadditional rock underlying the molten film to form a cut in the tunnelface. The plasma-jet provides a gaseous contact electrode thatfacilitates maintaining the electrically conductive path between thepower supply and the film of molten rock. The cooperating electrode maytake a number of forms, for example it may be a graphite rod, aconductive metal rod, or a second plasma-jet may be employed. The natureof the power supply. is a function of the characteristics of the rock tobe cut and may be DC or AC or DC with a superimposed AC signal. It ispreferred that the plasma-jet have a velocity in the order of at leastthree thousand feet per second. The dynamic erosive effects of this highvelocity jet facilitates the cutting operation. When the cooperatingelectrode is formed by a second plasma-jet, that jet in a particularembodiment has a substantially lower velocity, preferably less thanabout percent of the velocity of the other plasma-jet. It may also bedesirable in particular cutting operations to modify the characteristicsof the molten film by use of an additive such as an alkaline flux (e.g.sodium carbonate) with rock such as granite or an acid flux (e.g.potassium pyrosulfate) with rock such as basalt and such additive may beintroduced to the molten film by means of the plasma stream. Anauxiliary quench or ejection jet may be employed to facilitate removalof debris from the cut. Still another object of the invention is toprovide novel and improved methods for fragmenting rock.

Another aspect of the invention features breaking off fragments of rockby directing a plurality of plasma streams into holes or slots spacedapart in a region of the rock mass to make electrical contact betweenthe plasma streams and the rock faces contacted by the plasma streams,and then applyingelectrical power of a frequency effective for producingdielectric heating in the rock through saidplasma streams to heatdielectricly the rock mass between the rock faces to produce a heatedregion within the rock mass and thermal stress cracks that sever therock mass into fragments.

The plasma streams employed in this aspect of the invention arepreferably of low velocity so that a large electrical contact area isprovided at the bases of the spaced slots or holes. The applieddielectric heating power creates a heated core a substantial distancebelow the face of the rock mass so that greater amounts of rock areremoved than would be the case where the heated core was nearer thesurface.

The invention provides efficient methods for cutting and breaking rock.Other objects, features and advantages of the invention will be seen asthe following description of particular embodiments progresses inconjunction with the drawings in which:

FIG. 1 shows the invention applied to extending a tunnel face;

FIG. 2 shows, at larger scale, cutting according to the invention with apair of plasma streams being used to cut a slot into the rock of thetunnel face;

FIG. 3 shows a fragmenting operation according to the inventionemploying the application of plasma streams to the tunnel face; and

FIG. 4 shows a cross-section view through the center of a slot to revealin greater detail aspects of the cutting method shown in FIG. 2.

DESCRIFTION OF PARTICULAR EMBODIMENTS As shown in the drawing,tunnelling equipment 10 is brought up to face 12 of tunnel 14.Conventional support mechanism 16 supports torch holder 18 in positionbefore tunnel face 12. According to the invention the advance of thetunnel face is accomplished in two operations successively applied. Thefirst of these is a cutting operation shown more particularly in FIGS. 2and 4. During the cutting operation, torch holder 18 supports primaryplasma torch 20 and secondary plasma torch 22 in fixed relationship toeach other with the plasmas 24, 26 from each torch directed against therock face 12 to form an elongated slot 28.

A suitable primary plasma torch 20 is shown diagrammatically in FIG. 4and has a central tungsten cathode 32 supported by insulator 34 incavity 36 within housing 38 that carries anode electrode 40. Anode 40 iswater cooled and defines a nozzle passage 42. A conduit 44 connected toa supply 46 of inert gas such as argon communicates with cavity 36 toadmit gas symmetrically thereto for flow past cathode 32 and out throughnozzle 42. Pilot arc DC power supply 48 (which optionally may have asuper-imposed high frequency, e.g. 10 kHz) is connected with itsnegative pole to cathode 32 and its positive pole to anode 40. One ormore ports 50 into nozzle passage 42 provide a means for introducing anadditive into plasma-jet 24. Conduit 52 is connected to ports 50 todeliver the additive to torch 30.

A suitable secondary plasma torch 22 is also shown diagrammatically inFIG; 4 and has housing structure enclosing chamber 62 with a watercooled electrode 64 that defines an outlet orifice 66. Central electrode68 is supported within chamber 62. lnert gas is supplied through passage70 to chamber 62 for flow around electrode 68 and out through orifice66. A DC power supply 72 similar to power supply 48 is connected toelectrodes 64 and 68. A variety of other plasma torch configurations mayalso be used. For example, a porous anode torch of the type shown in US.Pat. No. 3,214,623 may be employed in particular applications as thesecondary plasma torch.

A cutting operation is initiated by turning on the flow of inert gas(i.e. argon) which enters chamber 36 of torch through conduit 44 andthen passes out through orifice 42. DC power supply 48 is turned on,causing pilot are 54 to form between cathode 32 and anode 40. The gasissuing from orifice is ionized by arc 54 to form a plasma. The gaspressure in chamber 36 is maintained at sufficient pressure so that ahigh velocity plasma-jet 24 issues from orifice 42. Typical velocitiesare in the range of 5,000 ft./sec. and above and preferably above 10,000ft./sec. Plasma-jet 24 is initially directed at the rock face 12 until aportion of the rock is melted to form a molten film 80. The electricalconductivity of this molten rock is higher than that of the solid rock.Secondary torch 22 is put in operation by flowing argon gas through thetorch and applying electric power from the power supply 72. Pilot arc 74is established between the central electrode 68 and the electrode wallof orifice 66. The diameter of orifice passage 66 is greater than thediameter of orifice 42, passage 66 has greater length, and the pressurein chamber 62 is less than that in chamber 36 of primary torch 20 sothat a relatively low velocity plasma stream 26 issues from torch 22.The plasma stream 26 from torch 22 is directed against and makeselectrocal contact with a portion of molten film 80 formed by the plasmajet 24 from primary torch 20. Power supply 84, which in a particularembodiment is a DC power supply of the welding type with a droopingcharacteristic, but which may provide AC or AC superimposed on DCdepending on the particular application, is switched on and atransferred arc path 86 is established with the current passing alongplasma arc 24, the molten rock 80 and plasma stream 26 which provides areturn electrode for the current, conducting it back to torch 22 and tothe power supply 84. A magnetic field usefully may be employed onelectrode 64 to rotate the contact point of the main current carryingarc 86 in particular applications. The electric current passing throughthe film 80 of molten rock strongly heats the film so that additionalrock underlying the current path is melted. The high velocity jet 24from torch 20 blows away material and thus enhances the rock cutting.Jet 88 (e.g. compressed air or a quench liquid) may be employed tofacilitate removal of debris from the slot 28. After the rock cuttingoperation has been initiated, torches 20 and 22 are advanced in fixedrelationship to each other by mechanism 16 across the face 12 of thetunnel to form the slot 28 in the face. Further slots are cut in thesame manner to produce a pattern of slots penetrating into the face adistance which may typically be in the order of one foot or more.

After a pattern of slots is cut into the tunnel face as described above,the blocks of rock are fragmented off the face. This is done with a pairof plasma torches 90, 92 as shown diagrammatically in FIG. 3. Apparatusfor the cracking operation is shown in FIG. 3. Torches 90 and 92 aresimilar to torch 20 described above except they are constructed forlower velocity operation to produce a long gaseous plasma column thatextends to the base of the hole or slot in the rock face. As theresistance of an argon column is in the range of 0.1 ohm cm., littlepower is dissipated in the gaseous electrode column. In particularapplications, it may be advantageous to use more than two gaseouselectrodes.

When in operation, torches and 92 are supported in front of rock face 12by torch holder 18. Suitable gas supplies, and power sources(diagrammatically indicated at 94) are connected to torches 90 and 92.Each of the torches 90 and 92 is put into operation in a manneressentially identical with that described above for torch 20. The plasmastreams 96, 98 issuing from torches 90 and 92 are directed into twospaced slots of the pattern. Each of the plasma streams establisheselectrical contact on the rock face of the slot along which it flows andits low velocity maintains an enlarged plasma environment at the base ofthe slot. The velocities of these streams should be in excess of twentyfeet per second and velocities in the range of 100-200 feet per secndare satisfactory in a particular embodiment. Electrical power of afrequency effective for dielectricly heating rock (e.g. a frequency inthe general range of 0.5 to 20 MHz) is applied across the two torches90, 92 and through the plasma stream conductors 96, 98 to the rock faceadjacent to the plasma stream. The electric field applied across therock (preferably at a voltage gradient in the range of 7507,500volts/inch) produces a thermal nugget 100 in the center of the rockblocks which creates thermal stress and fragments rock from the face ofthe tunnel. The fragmenting process is repeated at new locations in theslot pattern, thus extending the tunnel face into the rock mass. Afterthe face has been cleared by the fragmenting process, the cuttingprocess is resumed to further advance the tunnel face.

As an example of the principle of transferred arc cutting of rock, astandard TAFA Model 51 torch was connected to a 40 kW DC, 160 volt opencircuit power supply. The torch had a nozzle diameter of one-quarterinch and was located approximately inch above the face of a granite rockmass. A graphite rod was employed as the secondary or return electrode.Once the non-transferred arc was turned on and the rock became molten,the arc appeared to conduct through the molten material to the graphiterod. Typical operating parameters were:

DC volts 90 DC amps 400 Argon, gas flow (SCFH) The jet velocity usedduring these tests was in the range of 5,000 fps. The cutting speed wasabout 2 inches per minute and over 5 cubic inches of rock were removedper kWl-l consumed Thus it will be seen that the invention providestechniques employing one or more gaseous electrodes for cutting and/orbreaking hard rock. While particular embodiments of the invention havebeen shown and described, various modifications thereof will be apparentto those skilled in the art and it is not intended that the invention belimited to the disclosed embodiments or to details thereof, anddepartures maybe made therefrom within the spirit and scope of theinvention as defined in the claims.

What is claimed is:

l. The method of extending a tunnel face through a rock mass comprisingthe steps of cutting a pattern of slots into said tunnel face, each slotbeing cut by a method including directing a primary plasma stream from aprimary plasma torch to impinge on the tunnel face,

melting with said primary plasma stream portion of rock of the tunnelface to produce a molten film thereon,

contacting said molten film with a cooperating electrode, and

applying electrical power to said torch and cooperating electrode tocause current to flow along a path that includes said primary plasmastream, said molten film, and said cooperating electrode to further heatsaid molten film and melt additional rock underlying said film to form acut in the tunnel face, and

breaking off from the tunnel face fragments of rock between said slots,said fragment being broken off by a method including directing a firstplasma stream against a first region of said rock mass at the bottom ofa first slot cut in said tunnel face to make electrical contact betweensaid first plasma stream and a first rock surface,

directing a second plasma stream against a second region of said rockmass at the bottom of a second slot spaced from said first slot to makeelectrical contact between said second plasma stream and a second rocksurface,

and applying a voltage of a frequency effective for producing dielectricheating in said rock across said first and second plasma streams to saidfirst and second rock faces to dielectrically heat a portion of saidrock mass between said first and second rock faces to thermally stressand sever said rock mass portion into fragments.

2. The method as claimed in claim 1 wherein said primary plasma streamhas a velocity in the order of at least three thousand feet per second.

3. The method as claimed in claim 1 wherein said cooperating electrodeis a gaseous plasma stream.

4. The method as claimed in claim 3 wherein the velocity of saidcooperating plasma stream is less than about of the velocity of saidprimary plasma stream.

5. The method as claimed in claim 1 wherein said primary plasma streamhas a velocity substantially greater than the velocities of said firstand second plasma streams.

6. The method as claimed in claim 1 wherein the voltage applied acrosssaid first and second plasma streams and said rock mass portion has agradient in the range of 7507,500 volts/inch.

7. The method as claimed in claim 1 and further including the step ofsubjecting said molten film to a fluid jet to remove material from saidcut.

8. The method as claimed in claim 1 and further in cluding the step ofintroducing an additive to said molten film to modify thecharacteristics thereof.

9. The method as claimed in claim 8 wherein said additive is introducedto'said molten film via a plasma stream.

10. The method for cutting rock comprising the steps of energizing aprimary plasma torch to generate an elongated plasma stream,

directing said elongated plasma stream to impinge on the rock, melting aportion of the rock with said elongated plasma stream to produce amolten film thereon,

providing cooperating electrode means adjacent said molten film and insaid elongated plasma stream at a point beyond said molten film so thatsaid plasma stream extends along said molten film between said primarytorch and said cooperating electrode means, and

applying electrical power to said elongated plasma stream andcooperating electrode means to estalish a transferred are through saidplasma stream between said primary torch and said cooperating electrodemeans to cause electrical current to flow along a path between saidprimary torch and said cooperating electrode means through said plasmastream to further heat said molten film and and additional rockunderlying said molten film to form a cut in the rock.

11. The method as claimed in claim 10 wherein said cooperating electrodemeans is a gaseous plasma stream and said cooperating plasma stream isdirected to impinge on said molten film.

12. The method as claimed in claim 10 wherein said primary plasma streamhas a velocity in the order of at least three thousand feet per second.

13. The method as claimed in claim 10 and further including the step ofsubjecting said molten film to a fluid jet to remove material from saidcut.

14. The method as claimed in claim 10 and further including the step ofadvancing said primary plasma torch and said cooperating electrode meansin a fixed relationship to one another to extend said cut and form aslot through said rock.

15. The method as claimed in claim 10 and further including the step ofintroducing an additive to said molten film to modify thecharacteristics thereof.

16. The method as claimed in claim 15 wherein said additive isintroduced to said molten film via a plasma stream.

17. The method as claimed in claim 16 wherein said primary plasma streamhas a velocity in the order of at least three thousand feet persecond,said cooperating electrode means is a gaseous plasma stream and saidcooperating plasma stream is directed to impinge on said molten film andthe velocity of said cooperating plasma stream is less than about 10percent of the velocity of said primary plasma stream.

18. The method as claimed in claim 17 and further including the step ofsubjecting said molten film to a fluid jet to remove material from saidcut.

19. The method as claimed in claim 17 and further including the step ofadvancing said primary plasma torch and said cooperating electrode meansin a fixed relationship to one another to extend said out and form aslot through said rock.

20. The method of breaking off rock fragments from a mass of rockcomprising the steps of second plasma stream between said second torchand said second region, said first and second regions being spaced onefrom another,

and applying AC voltage of a frequency effective for producingdielectric heating in said rock across said first and second plasmastreams and the rock mass between said spaced regions to dielectricallyheat a portion of said rock mass between said spaced regions tothermally stress and sever frag- 20 feet per second.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. q 78870q Dated Januarv 29 197 i Invent r( Merle L Thorne It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column i, line 22, change "secnd" to secondh Column line 57, after"consumed" insert a period.

Column 6, line 13, "are" should be --arc-.

Signed and sealed this 21st day of May l97 L- (SEAL) Atte st:

1313mm) 1= i.FLI-1}T'IJPL-JR,J 1. MARSHALL DAMN Attesting OfficerCommissioner of Patents F RM PO- 0 0 USCOMM-DC 60376-P69 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. q 738 70q DatedJanuarv 29 19? Invento Merle L Thoroe It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, line 22, change "secnd" to "second- L Column 4, line 57,after- "consumed" insert a period.

Column 6, line 13, "are" should be --arc.

Signed and sealed this 21st day of May 197A.

(SEAL) Attest:

EDWARD l hlfPLEilT IlHEE'i, J 2. C. l-TARSHA'LL DANN Attesting OfficerCommissioner of Patents FORM PO-105O (10-69) USCOMIWDC a76 P9

1. The method of extending a tunnel face through a rock mass comprisingthe steps of cutting a pattern of slots into said tunnel face, each slotbeing cut by a method including directing a primary plasma stream from aprimary plasma torch to impinge on the tunnel face, melting with saidprimary plasma stream portion of rock of the tunnel face to produce amolten film thereon, contacting said molten film with a cooperatingelectrode, and applying electrical power to said torch and cooperatingelectrode to cause current to flow along a path that includes saidprimary plasma stream, said molten film, and said cooperating electrodeto further heat said molten film and melt additional rock underlyingsaid film to form a cut in the tunnel face, and breaking off from thetunnel face fragments of rock between said slots, said fragment beingbroken off by a method including directing a first plasma stream againsta first region of said rock mass at the bottom of a firSt slot cut insaid tunnel face to make electrical contact between said first plasmastream and a first rock surface, directing a second plasma streamagainst a second region of said rock mass at the bottom of a second slotspaced from said first slot to make electrical contact between saidsecond plasma stream and a second rock surface, and applying a voltageof a frequency effective for producing dielectric heating in said rockacross said first and second plasma streams to said first and secondrock faces to dielectrically heat a portion of said rock mass betweensaid first and second rock faces to thermally stress and sever said rockmass portion into fragments.
 2. The method as claimed in claim 1 whereinsaid primary plasma stream has a velocity in the order of at least threethousand feet per second.
 3. The method as claimed in claim 1 whereinsaid cooperating electrode is a gaseous plasma stream.
 4. The method asclaimed in claim 3 wherein the velocity of said cooperating plasmastream is less than about 10% of the velocity of said primary plasmastream.
 5. The method as claimed in claim 1 wherein said primary plasmastream has a velocity substantially greater than the velocities of saidfirst and second plasma streams.
 6. The method as claimed in claim 1wherein the voltage applied across said first and second plasma streamsand said rock mass portion has a gradient in the range of 750-7,500volts/inch.
 7. The method as claimed in claim 1 and further includingthe step of subjecting said molten film to a fluid jet to removematerial from said cut.
 8. The method as claimed in claim 1 and furtherincluding the step of introducing an additive to said molten film tomodify the characteristics thereof.
 9. The method as claimed in claim 8wherein said additive is introduced to said molten film via a plasmastream.
 10. The method for cutting rock comprising the steps ofenergizing a primary plasma torch to generate an elongated plasmastream, directing said elongated plasma stream to impinge on the rock,melting a portion of the rock with said elongated plasma stream toproduce a molten film thereon, providing cooperating electrode meansadjacent said molten film and in said elongated plasma stream at a pointbeyond said molten film so that said plasma stream extends along saidmolten film between said primary torch and said cooperating electrodemeans, and applying electrical power to said elongated plasma stream andcooperating electrode means to estalish a transferred are through saidplasma stream between said primary torch and said cooperating electrodemeans to cause electrical current to flow along a path between saidprimary torch and said cooperating electrode means through said plasmastream to further heat said molten film and and additional rockunderlying said molten film to form a cut in the rock.
 11. The method asclaimed in claim 10 wherein said cooperating electrode means is agaseous plasma stream and said cooperating plasma stream is directed toimpinge on said molten film.
 12. The method as claimed in claim 10wherein said primary plasma stream has a velocity in the order of atleast three thousand feet per second.
 13. The method as claimed in claim10 and further including the step of subjecting said molten film to afluid jet to remove material from said cut.
 14. The method as claimed inclaim 10 and further including the step of advancing said primary plasmatorch and said cooperating electrode means in a fixed relationship toone another to extend said cut and form a slot through said rock. 15.The method as claimed in claim 10 and further including the step ofintroducing an additive to said molten film to modify thecharacteristics thereof.
 16. The method as claimed in claim 15 whereinsaid additive is introduced to said molten film via a plasma stream. 17.The method as claimed in claim 16 wherein said primary plasma stream hasa velocity in the order of at least three thousand feet per second, saidcooperating electrode means is a gaseous plasma stream and saidcooperating plasma stream is directed to impinge on said molten film andthe velocity of said cooperating plasma stream is less than about 10percent of the velocity of said primary plasma stream.
 18. The method asclaimed in claim 17 and further including the step of subjecting saidmolten film to a fluid jet to remove material from said cut.
 19. Themethod as claimed in claim 17 and further including the step ofadvancing said primary plasma torch and said cooperating electrode meansin a fixed relationship to one another to extend said cut and form aslot through said rock.
 20. The method of breaking off rock fragmentsfrom a mass of rock comprising the steps of directing a first plasmastream from a first torch against the surface of a first region of saidrock mass to make electrical contact through said first plasma streambetween said first torch and said first region, directing a secondplasma stream from a second torch against the surface of a second regionof said rock mass to make electrical contact through said second plasmastream between said second torch and said second region, said first andsecond regions being spaced one from another, and applying AC voltage ofa frequency effective for producing dielectric heating in said rockacross said first and second plasma streams and the rock mass betweensaid spaced regions to dielectrically heat a portion of said rock massbetween said spaced regions to thermally stress and sever fragments fromsaid rock mass.
 21. The method as claimed in claim 20 wherein thevoltage applied across said first and second plasma streams and saidrock mass portion has a gradient in the range of 750-7,500 volts/inch.22. The method as claimed in claim 20 wherein the velocity of each ofsaid plasma streams is in excess of 20 feet per second.